1. Field of the Invention
The present invention relates generally to the field of video communications and is more particularly directed to a method and apparatus for conversion of a video signal from a first standard to a second standard.
2. State of the Prior Art
The problem of converting a video signal from one television standard to a different television standard, where the two standards differ in the number of scan lines per frame and/or in the field frequency has been addressed by a number of patents.
U.S. Pat. No. 4,283,736 issued to Morio et al on Aug. 11, 1981 discloses apparatus for converting both the field frequency and the scan line rate of a video signal. U.S. Pat. No. 4,276,565 issued to Dalton et al on June 30, 1981 also discloses a method and apparatus for standard conversion of television signals wherein conversion to either a higher or lower number of scan lines is possible. U.S. Pat. No. 4,240,101 issued Dec. 16, 1980 to Michael et al also teaches television standards conversion. Also known to the applicant, are U.S. Pat. Nos. 3,073,896 to James issued Jan. 15, 1963, U.S. Pat. No. 3,925,606 to Wood et al issued Dec. 9, 1975, and U.S. Pat. No. 3,917,906 to Johnson et al issued Nov. 4, 1975. The James reference teaches a digital conversion system for converting video signals from one format to another and further achieves a reduction in the bandwidth necessary to transmit the video information. Johnson et al discloses a system for multiplexing multiple information channels adjacent to the video spectrum on a television, microwave or cable unit so as to enable the transmission of additional information by modulation of subcarrier frequencies. Wood et al teaches a system for television display of periodic signals such as sonar returns for display on a conventional television monitor.
The approach taken by the above references to the problem of conversion of video signals from one scan line and field rate standard to a different standard has been uniformly based on digital techniques requiring digital storage devices and leading to costly and complex encoder-decoder systems.
The present trend in the video industry is towards higher resolution video imaging and development of high definition television (HDTV) systems. Proposed HDTV systems have an image format of 1,125 horizontal scanning lines, a bandwidth of 30 Megahertz, 30 frames per second, and an aspect ratio of 3 to 5 or 3 to 6.
Presently used home television receiving sets constructed for the American NTSC video standard are limited to an image format of 525 scanning lines, a 4.2 Megahertz bandwidth, 30 frames per second and a 3 to 4 aspect ratio. The existence and prospective widespread commercial use of such high definition television systems raises the problem of incompatability between the high definition signals and the capabilities of existing television receiving and video recording equipment in use today.
A need, therefore, exists for a line standard conversion system and particularly for one that makes possible a simple, inexpensive decoder which can be mass produced and distributed at low cost to the viewing public for use with existing video recording and receiving equipment. This need has not been met by presently available systems.
It is also desirable to minimize the signal bandwidth required by the video signals and particularly by high definition video signals, so as to conserve available broadcast frequency spectrum and minimize the bandwidth handling requirements of video equipment. While various bandwidth reduction techniques are known, applicant is not aware of any such methods which are consistent with a low cost decoder.